What does the idiom "an armchair strategist" mean and the source of allusions?

During the Warring States Period, Zhao She, a general of the State of Zhao, was promoted to Shangqing by King Huiwen of Zhao who was defeated and invaded Qin Jun. He had a son named Zhao Kuo, who was familiar with the art of war since childhood and loved to talk about military affairs. People often say that he is not. So he is arrogant and thinks he is invincible in the world. However, Zhao She was very worried about him, thinking that he was just an armchair strategist, and said, "In the future, Zhao doesn't need him as a general. If you use him as a general, Zhao Jun will be defeated. " Sure enough, in 259 BC, when he attacked again, Zhao insisted on resisting the enemy in Changping (now near Gaoping County, Shaanxi Province). Zhao was already dead at that time. Lian Po is in charge of commanding the whole army. Although he is old, he still has a good way of fighting, which makes Qin Jun unable to win. Knowing that it was not good for him to drag on, the State of Qin used double agents to send people to Zhao to spread the news of "General Zhao Kuo, the son of Zhao She, which Qin Jun was most afraid of". The prince of Zhao was cheated and sent Zhao Kuo instead of Lian Po. Zhao Kuo thought he was good at fighting, and moved all the articles in Sun Tzu's Art of War to the dead. After Changping, he completely changed Lian Po's battle plan. As a result, more than 400,000 Zhao troops were completely annihilated, and he himself was shot and killed by Qin Jun.